Microwaves, especially millimeter waves, are increasingly used, inter alia, for full body scanners. Millimeter waves are suitable for the detection of metallic and also nonmetallic objects, for example, ceramic objects such as knives. At the same time, air and other materials, such as are used for clothing, are transparent for such radiation. Accordingly, millimeter waves can be used for contactless scanners for the detection of objects concealed under the clothing. Such scanners are superior to metal detectors and can supplement or replace the latter, for example, in the field of security checks at airports or security-relevant public facilities.
In the operation of such a scanner, a microwave signal with a given bandwidth is generated and transmitted from at least one antenna in the direction towards a human body with objects concealed, optionally under the clothing. The microwave signals are reflected from the body and from the concealed objects. The reflections are received by at least one antenna. Transmitting and receiving antenna can be provided separately.
A scanner can also comprise a plurality of transmitting and/or receiving antennas. Such a scanner can be operated, for example, in a multi-static mode in which a transmitting antenna transmits a signal, and reflections of this signal are received according to amplitude and phase position by several or all receiving antennas. In another multi-static mode, several transmitting antennas can also transmit simultaneously, whereas the reflected signal of each individual transmitting antenna is received in each case by a plurality of receiving antennas.
Processes of aperture-synthesis can be used, for example, processes of digital beam forming (“Digital Beam-forming”, DBF), in which a focusing of the transmitted and/or received radiation can be implemented for the object scanning through lenses or similar without mechanical movement of the antennas and without electromagnetic bundling; instead, a focusing takes place only through targeted control of the transmitting antennas and/or evaluation of the signals detected by the receiving antennas. In the latter case, a signal reflected from a given spatial point in the direction towards several antennas and received there is evaluated, for example, through a software-based algorithm.
The German patent publication DE 10 2011 078 539 A1 describes a device for the illumination of an object under investigation, where the device comprises at least one reflector element for the reflection of microwave signals. In addition to the rays radiated directly onto the object and reflected from the latter, further microwave signals travelling via the reflector element are received and used for the reconstruction of the image of the object under investigation. For the synthetic focusing, the representation of one or more virtual transmitting or receiving antennas, whose positions correspond to the real positions of real transmitting respectively receiving antennas reflected on an angle of reflection, is assumed here.
The reflector element allows an improved illumination of object regions which are illuminated too little or not at all by directly radiated signals. However, reflections of the object caused by the reflector element can lead to disturbances in the reconstruction, that is, the imaging of the object.
What is therefore needed is an approach for imaging an object in which disturbances in the reconstruction caused by a reflection element can be minimized, and accordingly, an improved illumination and also an improved imaging quality can be achieved.